A wide variety of coatings have been used to coat the surfaces of packages such as food and beverage containers. For example, metal cans are sometimes coated using coil coating or sheet coating operations, that is, a coil or sheet of steel or aluminum is coated with a suitable composition and cured. The coated substrate is then formed into the can body or can end. Alternatively, the coating composition may be applied, for example, by spraying and dipping, to the formed can and then cured. Coatings for food and beverage containers should preferably be capable of high speed application to the substrate and provide the necessary properties when cured to perform in a demanding end use environment. For example, the coating should be safe for food contact and have excellent adhesion to the substrate.
To achieve the necessary coating properties, the coatings are thermosetting in nature, that is, the coating composition is based on a resinous binder that contains a polymeric material that contains active hydrogens, for example, a (meth)acrylic polymer or a polyester, each of which contain hydroxyl groups, and a curing agent that is reactive with the active hydrogens to form a crosslinked or thermoset coating. The curing agent of choice is a phenol-formaldehyde or an amine such as melamine, benzoguanamine or urea-formaldehyde condensate. Unfortunately, such curing agents release formaldehyde during the curing or crosslinking reaction.
This formaldehyde can be a strong irritant when allowed to accumulate in an enclosed space such as, for example, a curing oven. It is also suspected of being hazardous to the health of humans when allowed to accumulate to abnormally high levels in the ambient atmosphere. It is, therefore, desirable to eliminate formaldehyde emissions during the curing operation. One way to do this is to use an alternative curing agent that does not release formaldehyde yet provides the necessary properties for coating food or beverage containers such as adhesion, flexibility and resistance to acidic foods or beverages such as tomatoes and isotonic beverages.